(1) Field of the Invention
The present invention relates to a method for thermally treating a photoresist film, and more particularly to a method for forming in a resist film, which is sensitive to ultraviolet (UV) rays, a pattern in an undercut form upon the subsequent exposure and development process.
(2) Description of the Related Art
As a method for forming a metal electrode by a photo-lithography technique using a resist film, a Japanese Patent Application Kokai Publication No. Hei 2 -39613 has disclosed "a method for forming a fine pattern", which is shown in FIG. 1. As shown therein, a resist film 12 is deposited on a surface of a substrate 11 and this film has been selectively exposed in a desired pattern. A back surface of the substrate 11 is held on a vacuum chuck 13 which is maintained at a temperature of 20.degree.C. The resist film 12 is placed in a vessel 14 equipped with an agitator and a temperature controller and is positioned in such a way that the resist film 12 is in contact with a surface of the developing solution 15. It is so arranged that the resist film 12 is given temperature differences in the direction of the thickness of the film and that, after the development, an opening is formed in an undercut (overhang) form.
In the conventional method described above, as a method for forming the pattern in an undercut form in the resist film during the developing process, the temperature differences are created between the developing solution touching the surface of the resist film and the vacuum chuck holding the substrate. Due to the facts that, during the developing, the developing solution touching the resist film is kept at a constant temperature, that the thickness of the resist film is normally about few .mu.m and that the time required for the developing is in a matter of minute, the temperature of the surface of the resist film touching the developing solution becomes substantially the same as that of the developing solution itself. Therefore, a problem is that, if the pattern in an undercut form is attempted to be formed, it is necessary that the developing process be limited to a very short period of time.